Three-dimensional atomic force microscopy

3D atomic force microscopes can measure critical dimensions, line edge roughness and sidewall roughness in a way that is highly accurate, non-destructive and cost-effective.

One of the most challenging features in the semiconductor industry is the continuous research and the subsequent fabrication of integrated circuits with enduringly smaller critical dimensions (CDs). As shown in FIGURE 1, CDs must be measured at the top, middle and bottom of features, as well as various parameters such as line edge roughness (LER), the line width roughness (LWR) and the sidewall roughness (SWR).

The characterization of such factors that determine the shape and the roughness of the device patterns for device manufacturers is of utmost importance due to the fact that they directly affect the device performance. Optical measurement techniques, which are limited in terms of resolution. Therefore, the existing prevalent method for measuring these factors prior was primarily the scanning electron microscopy (SEM) with its image analysis software. Despite the fact that this technique offers substantial advantages such as automation and compatibility with standard critical dimension SEM tools, it cannot provide the user with high resolution LER data due to the fact that SEM resolution is reaching its limits, therefore 3D AFM offers a highly desirable solution. Leading manufacturers have implemented AFM that can measure resist profile, LER and SWR in a way that is highly accurate, non-destructive and cost-effective. The precise and full characterization of such features is extremely essential during the pattern transfer process as it offers the possibility of imaging all surfaces of the pattern.

FIGURE 1. LER, LWR and SWR are the limiting factors of resolution in optical lithography.

What is non–contact 3D AFM?
The basic principle of non-contact 3D-AFM is that a cantilevered beam rapidly oscillates just above the surface of the imaging sample. This offers several advantages, as compared to the traditional contact and intermittent modes. One of the advantages is that there is no physical contact between the tip and the surface of the sample. Moreover, as depicted in FIGURE 2, the Z-scanner, which moves the tip, is decoupled from the XY scanner, which solely moves the sample, thus, offering flat scanning and an additional benefit of improved Z-scan bandwidth. Furthermore, by tilting the Z-scanner, the sidewall of the nanostructures can be accessed and roughness measurements performed along the sidewall of photoresist lines. At the same time, measurements of the critical dimensions of top, middle, and bottom lines can be made.

FIGURE 2. The independent tilted Z-scanner enables measurements of the sidewalls of features.

FIGURE 3. Combination of the three acquired images for 3D AFM pattern reconstruction.

Data acquisition is performed by a conical tip in predefined tilted angles, typically 0º, a, and -aº. Consequently, and by combining these three scans (a method called image stitching), the 3D pattern can be constructed, as shown in FIGURE 3. This provides an excellent and extremely accurate method that takes advantage of the interference pattern of the standing waves in order to measure features such as the total height, the top, middle, and bottom width. 3D AFM is capable of advanced three-dimensional imaging of both isolated, and dense line profiles. It is less costly than the alternative techniques (CD-SEM and focused ion beam (FIB)) for imaging and measuring parameters of line profiles since the preparation of the sample is by far simpler.

Noise levels in 3D-AFM
A critical requirement when dealing with metrology tools is associated with constraining the level of noise in the manufacturing environment. A study of noise levels on a 300 mm wafer (FIGURE 4) shows the overall 3D AFM system noise at levels are lower than 0.05 nm (0.5 angstrom).

Roughness measurements
Roughness can be transferred into the final etched profile, thus, roughness measurements can describe and determine the quality of the patterns. The tilted Z scanner in combination with the low noise levels that are prevalent during the AFM process can provide accurate results in terms of sidewall roughness measurements. FIGURE 5 depicts the 3D AFM imaging of a photoresist semi-dense line pattern and the respective grainy structure of its sidewall. The precision with which the SWR was measured is validated by the high repeatability (0.08nm 1 sigma for 5 sites wafer mean) for the sidewall roughness of about 6.0 nm.

FIGURE 4. 3D AFM noise levels on a 300 mm wafer. The system noise level is less than 0.05 nm at every position and typically 0.02~0.03 nm RMS.

FIGURE 5. 3D AFM image of a photoresist semi-dense line pattern imaged with Z-scanner tilt. The bottom figure clearly depicts the grainy structure of the sidewall.

It needs to be noted that roughness depends, amongst others, on the aerial image contrast (AIC) or in other words the physics of exposure. AIC is determined as the quotient between the subtraction and the addition of the maximum and minimum image intensities.

Several consequent series of images with variable exposure reveal that LER significantly increases when the AIC is decreased, a fact that underlines that AIC is a controlling factor for LER. Moreover, and as depicted in FIGURE 6, reduced levels of AIC produced line profile images of the resist that were more blunted, and also smaller sidewall angles (SWA).

FIGURE 6. Park 3D AFM line profiles at different AIC levels reveal the proportionate relationship between SWA and AIC.

FIGURE 7. A 3D AFM image of a 300 nm photoresist line pattern yields full information regarding the morphology of the sidewall (top) Side-wall Roughness is different at different AIC levels, a fact that indicates the connection between LER and SWR (bottom).

FIGURE 7 illustrates the capability of Park 3D AFM to image all surfaces of the pattern, in contrast to the conventional AFM or the SEM, which cannot fully characterize the surface data, and obtain information such as base, top and both sidewall roughness from sidewall characterization. A 300 nm photoresist line pattern was imaged and the respective line profiles were obtained that clearly showed a substantial difference in terms of SWR between 97% and 40% AIC. More specifically, the lower the value of AIC, the more increased was the measured roughness. This intense decrease of roughness is underlying the fact that LER and the measured sidewall roughness are clearly correlated.

Finally, it needs to be emphasized the role of non-contact 3D AFM in terms of preserving the tip sharpness of the cantilever. In an independent study, researchers performed 150 consecutive measurements using the same tip and the tip wearing proved to be minimal. This is a prominent feature of AFM that prevents the continuous costly replacement of the tip but also ensures that the sample will be viable and not damaged by the AFM cantilever. The preservation of the tip sharpness allows for continual measurements of high resolution roughness data.

Conclusions
The potentialities of the innovative, non-destructive imaging technique of 3D AFM has several advantages compared to conventional SEM systems. An independent and tilted Z-scanner overcomes the disadvantages of alternative metrology tools and measure parameters such as detailed sidewall morphology and roughness, and sidewall angle characterization that render the optimization and evaluation process easier and far more detailed. •

KEIBOCK LEE is president and general manager of Park Systems, Santa Clara, CA.

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